Vaillant do a cupboard Combi Boiler. Dimensions in mm: H 700 x W 390 x D 295 ecoFIT pure combi The cupboard-size combination boiler for any home
+1 Can add Vaillant vSmart app to control it. And some links: https://www.vaillant.co.uk/homeowners/products/ecofit-pure-combi-26176.html https://www.vaillant.co.uk/homeowners/products/vsmart/ https://www.vaillant.co.uk/homeowners/products/vaillant-vsmart-app-based-heating-control-25856.html
https://www.mrcentralheating.co.uk/ Has a drop down box's where you can enter the physical size of your required boiler,Best to have a decent boiler installed,compromises are seldom worth it